1. Field of the Invention
This invention relates generally to physiological electrodes and their associated systems, and more specifically, this invention relates to physiological electrode systems by means of which a multiplicity of physiological functions may be achieved, either individually or in combination, through a single disposable electrode set.
2. Description of the Prior Art
Development of an understanding of electrical signals generated in the body and the utility of electrical signals supplied to the body has led to the necessity of transferring electrical energy to and from the body of a patient for medical purposes. This transfer of electrical energy to and from the body of a patient is achieved by means of electrodes contacting the skin of the patient. These electrodes may be generally classified as physiological electrodes.
Instruments or devices that are utilized in connection with physiological electrodes may be divided into three broad categories--monitoring devices, stimulating devices and therapeutic devices. Examples of monitoring devices include cardioscopes, electrocardiographs and electrocardiograms (for ease of reference the term "ECG" will be utilized to mean all or any of these devices) for monitoring operation of the heart and impedance pnuemographs for monitoring respiration. Therapeutic devices include electrosurgical units (ESU) and various radio frequency (RF) and other relatively high frequency applicators to reduce pain and promote healing. In the stimulating device category, there are defibrillators (used to shock a patient from fibrillation, an asynchronous cardiac ventricular or fluttering made of contractions) and other direct current (DC) and low frequency applicators. The line between a therapeutic device and a stimulating device is not always clear, but for purposes of this discussion a therapeutic device shall refer to an instrument involving high frequency signals (approximately 100 Hz. and higher), while a stimulating device is hereby defined as one employing DC or low frequency (approximately 60 Hz. and lower) signals.
Most commonly, ECG electrodes are small (on the order of 1/2 inch) conducting plates from which an electrical connection to the patient's skin is achieved by means of a saline gel. Each electrode has its own individual electrical lead to the ECG and a total of from three to seven electrodes (even more in the case of some diagnostic testing) are utilized for cardiac monitoring. The electrodes are generally disposable so that they are discarded after a single use, while the leads are retained. New electrodes are usually connected to the leads by a snap-on connection.
There are a number of problems associated with ECG electrodes of this type. For one thing, the multiplicity of separate leads means that the leads are continuously getting twisted together, thus creating storing and handling problems. With the twisted leads, it is also a problem to assure that proper connections are effected, even with color coding or similar attempts to minimize erroneous connections. Also, movement of the leads creates electrical signals, possibly by a piezoelectric-type effect, which cause distortion of the ECG signal with what are commonly known as motion or cable artifacts. Further, voltage potentials between the electrodes can produce displacement of the baselines of the ECG signals or traces by an effect known as DC offset which can, in severe cases, preclude the obtaining of an ECG trace. Variations of the DC offset with time produces a drift of the ECG baseline that further complicates evaluation of the ECG signals. Still another problem associated with ECG electrodes is the existence of noise on the ECG trace occurring as a result of too high of an impedance between the electrode and the patient's body. The existence of too high an impedance is frequently compounded by the fact that the electrodes are too rigid to accurately conform to the portion of the body on which they are located, so that the area of contact between the electrode and the body is reduced, thus increasing the resistance or impedance (contact impedance) of the electrical circuit at that point. In most ECG electrode arrangements, the snap for connecting the lead to the electrode is right over the center of the electrode, so that any tension on the lead tends to lift the electrode from the body and hence increase the impedance. Fluctuations in the tension on the lead will also vary the contact impedance at the electrode-body interface by changing the pressure on the gel and thereby form another source of artifacts.
Conventional defibrillators utilize a pair of paddles to which handles are attached for an operator to press the paddles against the patient's body. A saline gel is placed on the paddles before they are applied to the patient to provide the desired interface between the paddles and the skin of the patient. As the paddles are pressed against the chest of the patient, a high voltage pulse of defibrillating energy is passed to the patient's body by actuation of discharge control buttons in the paddle handles.
One of the most disadvantageous features of the conventional defibrillator is that the operator is immediately adjacent the point of discharge. Thus, the risk that the operator will get shocked is not insignificant.
From the standpoint of efficacy, a major disadvantage of the conventional defibrillator paddles is that both paddles are applied to the chest of the patient. Testing has shown that for the best results in defibrillation it is desirable to have one of the defibrillating electrodes on the front of the body and the other on the back. Not only does this provide more current to the heart to increase the chances of a successful conversion (resuscitation by converting the heart from fibrillation to a life-sustaining rhythm), but it also reduces localized current densities, which test results suggest produces less myocardial damage. It is, of course, very difficult, if not almost impossible, in an emergency situation to prop a patient up so that one paddle can be pressed against the chest and the other against the back of the patient.
Yet another disadvantage of having both paddles on the chest of the patient is that a conducting path can be established over the skin of the patient from one paddle to the other, thus reducing the energy passed through the body tissue to the heart and also increasing the chances that a patient may be burned at the paddles. A further negative aspect of conventional paddles is that they are very difficult to apply to a patient that is draped for surgery or to whom a cardiopulminary resuscitation (CPR) device is attached. Still another problem with convention paddle defibrillators is that the paddle-to-skin impedance may be too high, thereby causing energy loss and increasing the risk of skin burns. A number of factors contribute to this undesirably high impedance, one of them being the rigidity of the paddles which prevents them from sufficiently conforming to the portion of the body to which they are applied. This problem may be overcome to some degree by pressure applied to the handles, but other factors such as insufficient area, less desirable contact metals and the use of low-quality gels still make the impedance problem one of concern. It may be noted that an insufficient paddle area also provides less desirable current density patterns.
Finally, conventional paddle defibrillators have the disadvantage that when a patient begins fibrillation the paddles must be gelled before being applied to the patient. The greater the time between the onset of fibrillation and the application of a defibrillating pulse, the greater the possibility that the patient will not be successfully converted.
Some of these problems with paddle defibrillators have been at least partially resolved by a disposable defibrillator electrode set known as the "BI-PAK" sold by Zenex Corporation. While this device deals with the basic problem of a front-to-back (anterior-posterior) electrode placement, the impedance characteristics may be improved upon. In addition, the "BI-PAK" does not provide for the connection of the electrodes to a separate ECG or to an ESU.
In order to return RF energy entering the body from an electrosurgical knife, an ESU return pad is normally placed under or attached to the patient, with a conducting lead extending back to the ESU return terminal. Various shapes and sizes of these return pads have been utilized, as well as a variety of conducting materials.
Since the RF currents introduced into the body during the electrosurgical process are relatively large, there is a continual problem of extracting this RF energy from the patient's body without heating the pad and burning the skin due to current concentrations at the ESU pad. Frequently patients are burned despite the efforts to preclude such a result. Such burns usually occur as a result of a non-uniform current density at various locations of the ESU return pad, particularly about the outer perimeter thereof, which is referred to as an improper dispersion of the RF current. Another problem of prior art ESU return pads that does not appear to be recognized is the existence of DC or low frequency shocking that occurs during electrosurgical operations. Some of this undersired shocking is probably due to leakage currents reaching the body through the ESU pad. However, it appears that some DC or low frequency currents are an inherent aspect of electrosurgical operations, due to rectification of the RF signals during tissue cutting. With a continuous DC or low frequency current path through the ESU return pad and the lead back to the ESU, it seems that there exists an ever-present danger of shocking the patient by undesired DC and low frequency signals during an electrosurgical operation.
Apart from the problems associated with the ECG, defibrillator and ESU electrodes individually, significant problems are encountered when more than one of these instruments is used at the same time. Thus, while the patient is being monitored by an ECG, fibrillation may commence and it is necessary to apply a defibrillating pulse of energy to save the patient. Similarly, during an electrosurgical operation, the condition of the patient's heart will frequently be monitored by an ECG. Further, it may also be necessary to defibrillate during the electrosurgical operation.
Pulses of defibrillation energy while ECG electrodes or an ESU return pad are connected to the patient may produce burns under the electrodes or pad, as well as damaging the ECG and ESU instruments. High voltage protection circuits have been utilized to prevent these occurrences. However, the recovery time for an ECG trace after application of a defibrillator signal may take anywhere from a few seconds to over a minute. Loss of the ECG trace at the time of defibrillation is particularly crucial, since it is imperative to know if the defibrillation shock was successful in terminating the fibrillation. Also, while the high voltage protection circuits protect the patient from burns and the ECG from damage, they also tend to prolong the recovery time for the ECG trace.
RF signals from the ESU create additional problems for the ECG, as these relatively high energy signals can create burns under the ECG electrodes, as well as significantly interfering with the ECG trace (especially by lower harmonic distortion). Filter circuits have been utilized to protect the ECG from such RF interference, but such filters frequently reduce the amplitude of the ECG trace so that it becomes difficult to analyze.
One of the primary problems occurring at the present time is that efforts have been directed to isolate the ECG, the ESU and the defibrillator from one another to prevent the problems referred to above. However, these attempts at isolation have precluded the instruments from having a common reference, so that an additional hazard is created by potential differences between the instruments themselves.